When developing marginal offshore production fields it is important to keep the costs low. A substantial cost factor with known forms of development, is due to subsea pipelines and cables between christmas trees at the wells and product receiver sites, for example a platform or a floating production vessel. Typically there may here be the question of distances like 2 km. In this connection it is to be noted that modern drilling technology makes it possible to produce by means of a reduced number of christmas trees, because the boreholes or wells can have several branches. This will make it possible to develop subsea fields in some instances by employing only one subsea christmas tree.
International patent application PC/NO96/00201 dated Jul. 8, 1996 relates to a system for offshore production of hydrocarbons by employing a moored production vessel or ship. The present invention can be regarded as a further development thereof and is based on the idea that the production vessel can by lying directly above or in the immediate vicinity of the christmas tree at the seabed and that the well fluid flows are transferred directly between the christmas tree and the vessel without relying on pipelines or cables on the seabed.
Advantageously the solution to be described in the following specification can employ a particular and preferably simplified form of christmas tree, being more or less integrated into an arrangement according to the invention as will appear from the following description.
Thus, in a device as stated in the introduction above, the novel and specific features according to the invention primarily comprise therein that the fluid swivel is provided with a rotatable swivel housing at the top of a stationary christmas tree, preferably comprising a small number of valves, such as a production master valve and an annulus master valve, and that said passages are through-running vertically in a central core member of the fluid swivel in order to make possible well intervention from the upper side of the fluid swivel.
On the basis of this fundamental solution, which in actual practice can be embodied in various forms, it is possible to obtain a number of advantages, of which in particular the following are mentioned:
The production vessel employed does not need any modifications worth mentioning, in relation to common ship designs, and therefore will be relatively inexpensive, PA0 the same production vessel can be used for installing and possibly retrieving the swivel device, or also for well maintenance, which contributes to reduced operational costs, PA0 very small marginal subsea fields can be profitable by using this novel arrangement, so that the degree of extraction can be increased for the fields, PA0 the equipment employed can be re-used by moving it from field to field.